This invention relates to a method and apparatus for inspecting cylindrically configured pellets for surface defects.
In the manufacture of nuclear fuel rods, nuclear fuel pellets, which are formed from a matrix of enriched or natural uranium oxide, are inserted into elongated, hollow rods typically formed from an alloy of zirconium.
The rods are sealed at the end with end plugs and the rods are pressurized. The rods are stacked in an array to from the core of a nuclear fuel reactor. Defects in the pellets, such as fissures or cracks, often produce chips during reactor operation which can adversely affect the nuclear fuel reactor operation. For example, during reactor operation, a loose chip dislodged from a nuclear fuel pellet could become lodged within the fuel rod adjacent the interior wall of the rod. During reactor operation, the fissionable material contained in the dislodged chip will continue its fissionable reaction and the heat generated during fission may create a defined area of intense heat on the rod wall. As a result, this localized area of intense heat will weaken the rod at the localized point and may cause a rupture in the rod wall creating a leak of the high pressure gas in the rod. If this occurs, the reactor core may have to be shut down Additionally, other pellet surface defects, such as the inclusion of metal in the pellet surface, are objectionable because the defects adversely affect the normal fission reaction of the uranium. As a result, fuel pellets must be inspected during their manufacture for unacceptable surface defects. Preferably, a pellet visual inspection process will have almost one hundred percent inspection validity for assuring defect free production of fuel rods.
In some nuclear fuel manufacturing plants, fuel pellets are manually inspected. Specially dressed inspectors view a single side of a tray of pellets. The pellets are stacked in the tray lengthwise. The tray is positioned under a light source for illuminating the pellets. The pellets are stacked end-to-end and the pellet-to-pellet interface is highlighted to facilitate the inspectors' locating of end defects and surface irregularities.
After one side of a tray is viewed, a lid is placed over the tray of pellets, and the tray inverted. The backside of the tray is removed to yield viewing access to the opposite side of the pellets. The inspector completes the inspection process by viewing the pellets as before.
This type of manual inspection suffers several drawbacks. Radioactive and hazardous dust often is present in the air. The inspectors must be protected against this dust with special protective clothing. Additionally, the prolonged visual inspection of the trays is inherently strenuous to the eyes which causes inspector error over prolonged periods of time. Also, uranium pellets are heavy and the continual inverting of the trays containing the pellets is tiresome to the inspector.
As a result of the danger involved in a manual pellet inspection system, it is more desirable to automatically inspect nuclear fuel pellets without relying on human involvement. Many automated pellet inspection systems, however, have suffered several drawbacks. Nuclear fuel pellets are extremely abrasive, and the pellet engaging surfaces of material handling systems tend to wear quickly. Additionally, if small chips are broken from off a nuclear fuel pellet during handling, they often damage the belts, rollers and gears of the material handling systems.
In one automated prior art pellet inspection apparatus, pellets are stacked end-to-end and illuminated and the interface between the pellets analyzed by appropriate optics and cameras. The stacking of pellets is undesirable because the interface between two contiguous pellets must be determined to distinguish between pellets. If the individual pellets vary in length from each other, the only method for separating individual pellets for inspection analysis is by finding the edge-to-edge interface and then segregating among the individual pellets. At the same time the edge-to-edge interface is determined, complete inspection of the pellet surface must occur and if a pellet is determined defective, the edge-to-edge interface often must be detected again to identify and reject the defective pellet. This type of system is complex. Additionally, because the pellets are stacked during inspection, complex material handling equipment is necessary to ensure proper feeding and stacking of pellets during conveyance and inspection. An example of such an automatic pellet inspection system as described above is disclosed in U.S. Pat. No. 4,496,056 to Schoenig, Jr. et al. As described therein, individual nuclear fuel pellets are conveyed in stacked relationship to each other to a pellet inspection area. The interface between the stacked pellets is located and the surface reflectivity of the pellets is analyzed to determine the cylindrical shape and length of the pellet. As is described in related U.S. Pat. No. 4,549,662 to Schoenig et al., the disclosed apparatus for effecting the inspection process is complex for ensuring the proper conveyance and delivery of a stack of pellets.
In another complex pellet inspection apparatus, disclosed in the U.S. Pat. No. 4,448,680 to Wilks et al., a nuclear fuel pellet is transferred to an array of stations for detecting at individual stations the diameter, length and surface flaws on the pellet. At the surface flaw detection station, a converging beam is cast on a fuel pellet by a rotating scanning prism. Flaws are detected by detecting alternating light and dark areas. However, this type of system does not provide for reliable analysis of defects such as metal inclusions which typically are visualized as brighter spots on the pellet surface than the surrounding pellet surface. Additionally, the system requires complex pellet handling systems.